1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a temperature detector.
2. Description of the Related Art
An SRAM has been conventionally used as a work memory of portable equipment such as a cellular phone. However, memory capacity necessary for portable equipment has been increasing year after year. Therefore, a DRAM having dynamic memory cells or a pseudo SRAM is adopted as a work memory in recent years. A DRAM can have larger memory capacity as a work memory with the same cost since a memory cell thereof is smaller than a memory cell of the SRAM.
Meanwhile, a memory mounted on a cellular phone needs to be low in power consumption in order to make the battery life long. A low standby current is especially important in a cellular phone in order to increase the continuous standby time. The DRAM and the pseudo SRAM require regular refresh operation of memory cells even while the portable equipment is in nonoperation, and the refresh operation is a factor of increasing the standby current. Therefore, various methods are devised in order to reduce the standby current in the DRAM and the pseudo SRAM.
For example, some technique has been proposed for reducing the standby current, by taking advantage of the characteristics of the dynamic memory cell that has the data retention time which gets longer as the chip temperature is higher, and setting a long refresh interval when the chip temperature is lower than a certain boundary temperature to reduce the frequency of the refresh operation (disclosed in, for example, Japanese Unexamined Patent Application Nos. Hei 5-266658, Hei 7-73668, and Hei 3-207084).
FIG. 1 shows temperature-dependent data retention time of a dynamic memory cell. As described above, the lower the chip temperature is, the longer the data retention time of the dynamic memory cell is. It is possible to reduce the standby current by changing the refresh interval according to the boundary temperature Tth detected by a temperature detector.
FIG. 2 shows a problematic example of a conventional semiconductor integrated circuit having a temperature detector. When the semiconductor integrated circuit operates around a boundary temperature Tth, an output of the temperature detector varies in a short cycle if heat generation due to the operation of internal circuits and heat release due to operation termination of the internal circuits are repeated. As a result, a control circuit connected to an output of the temperature detector changes its operating state (low power operation and normal operation) in response to the output of the temperature detector. This switching operation increases current consumption of the control circuit, so that an effect of reducing the standby current is lowered.
FIG. 3 shows another example of a problem of the conventional semiconductor integrated circuit having the temperature detector. When the operation and nonoperation of the internal circuits of the semiconductor integrated circuit are repeated around the boundary temperature Tth, the temperature detector sometimes malfunctions since it detects power supply noises as temperature variation. Therefore, the output of the temperature detector varies with a short cycle. At this time, since, as in FIG. 2, the operating state of the control circuit connected to the output of the temperature detector is switched over with a short cycle, current consumption of the control circuit increases. Further, the operating state of the control circuit shown in FIG. 3 is switched over irrespective of the chip temperature, so that the semiconductor integrated circuit malfunctions.